X-ray and gamma-ray radiation detectors have applications in science (e.g., astro- and radio-physics, radiochemistry, medical research, etc.) and industry (e.g., medical and dental diagnostics). Furthermore, world events have created new applications for these detectors for national security purposes e.g., detection of radioisotopes. Although radiation detection may be implemented using many different techniques, the use of wide band gap semiconducting materials is the highly preferred choice since they yield far better energy resolution. Despite the advantages of semiconductor materials, only a handful of suitable compounds have been identified thus far, due to the strict requirements on the properties they should exhibit. Even for the benchmark compound used at room temperature, Cd0.9Zn0.1Te, inherently related growth and fabrication problems have limited its widespread use in radiation detectors.
Wide band gap semiconductors can be used in applications with the most demanding energy resolution requirements since they offer compact and highly versatile configurations with unsurpassed energy resolution. The latter property is based on considerations of: a) signal intensity (for a radiation interaction of a fixed energy, E, it can be shown that essentially all the generated electrons and holes can be used as information carriers); and b) statistical accuracy of the produced signal (the theoretical statistical variance of semiconductors, described by the so called Fano factor, can, under specific conditions, be zero, while experimentally it can approach zero).
The energy of incident ionizing radiation on a semiconductor is partitioned through various interactions, involving quantum energy processes, to electrons, plasmons, photons and atomic nuclei. At the end of this complex combination of interactions, known as the energy cascade, electron-hole pairs are generated in proportion to the energy of the absorbed incident photons. The charge carriers can, subsequently, be read out and provide information about the intensity and energy of the incident radiation. FIG. 1 shows a schematic diagram of a simple planar radiation detector. Upon absorption of electromagnetic radiation in a semiconducting material 100, electron-hole pairs are generated through the different scattering mechanisms described in detail below. The generated electron-hole pairs are read out by applying a large electric field, E, across the semiconducting material using a pair of electrodes (i.e., an anode and a cathode) in electrical communication with the semiconducting material. In order to have an efficient read-out it is imperative that the generated holes and electrons reach the respective electrodes. Hence, the material must allow the charge carriers to diffuse to the electrodes without being scattered (i.e., the mobility as well as the lifetime of carriers is desirably as high as possible). Low noise on the read-out requires low leakage currents. This implies a band gap that is desirably larger than 1.5 eV in order to operate at room temperature so effects of thermally excited carriers are minimized. One of the measures of quality for a semiconducting material used in this respect is the electrical resistivity which is desirably in the region of several GΩ cm.
The extent to which the semiconducting material absorbs radiation depends on the atomic numbers, Z, of its constituent elements. In the region below about 200 keV the absorption is mainly due to the photoelectric effect which scales with Zn, where n typically is between 4 to 5. Compton scattering dominates until the MeV regime is reached, after which electron-positron pair (or their interation products) production becomes the principal scattering mechanism. The Compton scattering cross section scales linearly with Z whereas the cross section for pair production scales with Z squared. Hence efficient detector materials should contain elements with high Z values.
The benchmark material is Cd0.9Zn0.1Te (CZT), which is used in commercial as well as scientific devices operating at room temperature. Despite its relative simplicity, several problems are associated with this compound. For commercial purposes the material is synthesized using the high pressure Bridgman method due to the volatility of the elements at the melting point of the phase. This is a costly and complicated setup and growth times are 3-4 weeks for a single crystal of the desired quality per grown boule. The reason being that synthesized CZT contains a significant amount of macro defects such as grains, cracks and Te precipitates. Since Te is a narrow band gap semiconductor these precipitates increases leakage currents and interfere with charge transport. These macro defects significantly reduce the parts of the boule which are detector grade. Furthermore the material is a solid solution between ZnTe and CdTe, hence there is inherently a decreasing Zn content along the growth direction. This further reduces the detector grade part of the boule. As a result, less than 10% of a synthesized boule can be used to fabricate detectors. Another problem inherent to CZT is the low mobility of the holes in the compound. For this reason, detectors with electrodes designed to detect only electrons are generally used. This reduces the sensitivity compared to detectors designed to detect both carrier types.